1. Field of the Invention
This invention relates to hot melt adhesives based on elastomeric ethylene copolymers. In particular, the invention relates to hot melt adhesives especially comprising ethylene/alpha-olefin copolymers wherein the alpha-olefin can have from 3-20 carbon atoms such as ethylene/butene-1, ethylene/hexene-1, ethylene/octene-1, and ethylene/propylene copolymers. These ethylene copolymers with prescribed range of comonomer levels can be prepared by polymerization of the suitable olefins in the presence of supported or unsupported metallocene-alumoxane catalysts systems.
2. Description of the Prior Art
Hot melt adhesives are mixtures of polymer and adjuvants, usually tackifying resin, wax, or low Tg materials such as oils and low molecular weight polymers. The polymer contributes cohesive strength to the final formulation. It is well known that highly crystalline polymers show very poor adhesive performance at temperatures below room temperature due to brittleness, hence, somewhat elastic polymers are most useful in hot melt adhesives. In addition, the presence of the comonomer improves adhesion of the final hot melt adhesive formulation to metal and polar substrates such as aluminum and glass.
Copolymers of ethylene and a mono-olefinically unsaturated polar comonomer such as vinyl acetate, methyl acrylate, acrylic acid and the likes, have been reported as useful polymers to formulate into hot melt adhesives. These hot melt adhesives generally are lacking in elevated temperature performance. These hot melt adhesives, although useful with polar substrates, are less useful with non-polar substrates.
Hot melt adhesives prepared with EVA (ethylene/vinyl acetate copolymer) containing high levels of vinyl acetate are generally ineffective in laminating non-polar plastic surfaces to the same plastic surfaces or to other surfaces. For example, polypropylene is widely used for product assembly in automobile industry due to low cost and solvent resistance. The polypropylene parts need a hot melt adhesive to bond to other polymer parts or bond to other substrates such as steel, aluminum, glass, etc. Besides excellent adhesion, another highly desirable requirement of the hot melt composition is its elevated temperature properties. Besides product assembly applications, polypropylene is widely used in film and in cast sheet form for packaging articles and for packaging food and such packaging requires closure which can resist mechanical breakage.
U.S. Pat. No. 4,072,735 discloses the preparation of a hot melt pressure sensitive adhesive which comprises ethylene-propylene rubber, tackifying resin, polybutene, and, optionally, a crystalline polypropylene. The addition of the crystalline polypropylene is required in order to obtain a useful adhesive at elevated temperatures.
U.S. Pat. No. 4,568,713 discloses a hot melt adhesive system comprising butene-1/ethylene copolymer (containing 5.5-10.0 wt. % ethylene), an aliphatic/non-polar tackifier, an antioxidizing agent and, optionally, microcrystalline wax. The patent discloses utility for long open time and the need for good cold metal bonding to steel and aluminum.
JP-62-129303 discloses the preparation of ethylene/alpha-olefin copolymers using metallocene-alumoxane catalysts. While the products obtained are olefin copolymers having narrow molecular weight distributions and relatively low softening points, these copolymers are waxes characterized by only slight surface stickiness or slight adhesiveness, and are therefore used as pigment dispersing agents and toners.
JP-61-236804 discloses preparing narrow molecular weight distribution ethylene/alpha-olefin copolymers with metallocene-alumoxane catalysts, however, these ethylene/alpha-olefin copolymer products are also waxes that are used as pigment dispersants and resin processing aids. No adhesive characteristics are taught for these products.
JP-62-121709 pertains to the copolymerization of ethylene with an alpha-olefin in the presence of a catalyst comprising a zirconium hydride compound such as a bis(cyclopentadienyl) zirconium monochlorohydride and an alumoxane to obtain a copolymer which is narrow in both molecular weight distribution and composition distribution. However, while the copolymer is excellent in transparency, impact resistance and heat sealability, it is also characterized by surface non-stickiness.
There is an apparent need in the art to obtain hot melt adhesives having improved elevated temperature properties, and better low temperature flexibility.